Method of gold plating by chemical reduction



United States Patent 2,976,181 METHOD OF GOLD PLATING BY CHEMICAL REDUCTION Robert R. Brookshire, Canoga Park, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware No Drawing. Filed Dec. 17, 1957, Ser. No. 703,291

4 Claims. (Cl. 117-130) This invention relates to electroless plating by the deposition of metals by chemical reduction. More particularly the invention relates to improvement in the deposition of gold from chemical reduction plating solutions.

. reduction processes in contrast with conventional electroplating methods where the plating of such recesses is a practical impossibility. Known prior chemical reduction processes, however, all employ soluble metallic compounds which are exceptionally light sensitive and therefore easily reduced. It will be appreciated that the ease of reduction of such light sensitive compounds renders the process difficult to control. Generally'in order to achieve control it is necessary to carry out the processes under darkroom conditions or to include other additives in the plating solution which increase the solubility of the compounds and thus reduce the light sensitivity thereof. Attempts to increase the solubility of metallic compounds in aqueous solutions have not been markedly successful and such solutions have,-in general, failed to produce a good plate.

It is therefore an object of the instant invention to provide an improved method for plating suitable surfaces with gold by a controllable chemical reduction process.

According to the invention a chemical reduction plating solution is employed in which an insoluble compound of gold is included. Since an insoluble gold compound is employed, the plating reaction is not adversely afiected by light and greater control of the plating process is thus obtained. The reduction of the insoluble compound is made possible by including in the plating solution a chelating or complexing soluble compound which forms a soluble complex with the insoluble compound. This complex is then reducible upon a suitable surface by the action of a reducing agent.

Examples of metals and alloys which may be coated or plated with gold by the practice of the present invention include steel, iron, ferrous alloys, nickel, cobalt, gold, silver, platinum, copper, copper base alloys, magnesium and aluminum. It will be understood that superior gold plating results when the surface of the article to be coated is thoroughly cleaned. Hence, in most ulararticle. V 7

agent to the insoluble gold compound is not critical.

instances it is desirable to employ conventional plating pretreatments, such as butting, abrading, or acid etching prior to chemical reduction plating.

In the practice of a specific embodiment of the invention an improved gold deposit or plating is obtained by immersing an article having a surface to be plated into an aqueous solution in contact with water-insoluble gold cyanide, and containing a reducing agent such as sodium hypophosphite and a chelating or complexing agent such as sodium or potassium cyanide. The purpose of the complexing agent is to maintain a relatively small por- 2,976,181 Patented Mar. 21, 1961 a CC 2 tion of the gold in solution as a watersoluble gold complex while permitting a relatively large portion of the gold in the water-insoluble gold cyanide to remain out of solution as a gold reserve in contact with the aqueous plating solution. The reduction plating is continued until a predetermined thickness of plating is obtained or until the gold ion content in the solution and the reserve supply of gold cyanide are substantially exhausted. The function of the chelating or complexing agent is to continuously form a soluble complex with the insoluble gold compound and to continuously hold the gold in solution in a controlled concentration until reduced to the metallic state. During the'plating processes the plating solution bath is maintained at a temperature in the range of from about 50 C. to about C. At the higher temperature the plating rate is increased and becomes more difficult to control.

soluble gold compound and the soluble chelating or complexing agent to water. The amount of insoluble gold compound is determined by the amount necessary to deposit a desir ed thickness of gold on a particular surface, which amount can, of course, be calculated'for a partic- The ratio of the chelating or complexing However, it will be understood that if anion ratio of greater thanabout 10 to 1 chelating agent to insoluble compound is employed, the gold ion is held so strongly in solution by the chelating agent that its reduction therefrom becomes diflicult and the plating process becomes less controllable. Likewise employing a ratio of less than about 0.01 to 1 chelating agent to insoluble salt results in an extremely low rate of plating or no plating at all since there is an insufficient amount of chelating agent to form a significant amount of soluble complex. The hypophosphite reducing agent is then added to the solution, the quantity being determined by the amount of gold to be reduced. In general it is advisable to provide an excess amount of hypophosphite radical to accommodate side reactions and to maintain the plating reaction during the final reduction stages. It has been found that excellent results are obtainable when an ion ratio of hypophosphite radical to insoluble compound of 0333-10 to 1 is employed. The term ion as employed herein includes the total quantity of element or radical present in the bath, and expressed in ionic units, that is, dissolved and undissolved, unless otherwise described, and dissociated and undissociated. By the term ion ratio is meant the ratio of molar concentration of one ion relative to the molar concentration of a reference ion.

The following are illustrative examples of initial plating bath compositions and conditions in accordance with the present invention wherein the quantities, where applicable, are expressed in terms of grams per liter:

Average plating rate (mgsJcmP/hr.) 9.85

Example 2 AuCN 2.0

Na-H2PO2.H2O KCN p 0.2 Temperature C 96.0 pH 7 7.5 Work load (cmF/crnfi) 0.25 Average plating rate (ingsJcinF/hr.) 9.85

Example 3 AuCN 20.0 NEH2P03.H2O KCN 80.0 Temperature C 96.0 pH 13.5 Work load (cmF/cmfi) I 0.25 Average plating rate (mgsJcmF/hr.) 12.3

Example 4 AuCN 2.0 NaI-I PO .H O 10.0 NaCN 0.4 Temperature C-.. 96.0 pH 13.5 Work load (cm. /cm. 0.25 Average plating rate (mgs./cm /l1r.) 8.2

Example 5 AuCN 2.0 NaH P0 .H O 1.0 KCN 0.2 Temperature C 96.0 7.1 Work load (cmP/cmfi) 0.25 Average plating rate (mgs/cmF/hr.) 3.68

It will thus be understood from the foregoing description and examples of the invention that an improved method of plating gold by chemical reduction has been provided in which the reduction action is not sensitive to light and is more controllable whereby an improved plating of gold is obtained. It should be understood that, although the invention has been described with specific reference to particular embodiments thereof, it it not to be so limited since changes and alterations therein may be made which are within the intended scope of the invention as defined in the claims appended.

What is claimed is:

1. The method of gold plating comprising the step of contacting a surface to be plated with a plating solution containing a quantity of dissolved gold, a dissolved hypophosphite reducing agent, a gold cyanide complexin" agent, and a quantity of nndissolved gold cyanide in contact with the plating solution.

2. The method of gold-plating comprising the step of contacting the surface to be plated with an alkaline plating solution containing a dissolved alkali metal cyanide, a dissolved hypophosphite reducing agent, dissolved gold, and nndissolved gold cyanide in contact with the plating solution.

3. The method of gold-plating comprising the step of contacting the surface to be plated with a plating solution containing a dissolved alkali metal cyanide, a dissolved hypophosphite reducing agent, dissolved gold, and nndissolved gold cyanide in contact with the plating solution, the ion ratio of said hypophosphite ion to the undissolved gold ion being in the range of about 0.33 to about 10.

4. The method according to claim 3, wherein the ion ratio of the cyanide ion of the dissolved alkali metal cyanide to the nndissolved gold ion is in the range of about 0.01 to about 10.

References Cited in the file of this patent UNITED STATES PATENTS 744,170 Darlay Nov. 17, 1903 1,207,218 Roux Dec. 5, 1916 2,658,841 Gutzeit Nov. 10, 1953 2,726,969 Spaulding Dec. 13, 1955 OTHER REFERENCES Wein: Gold Films, The Glass Industry, May 1959, page 280; June 1959, page 330. 

1. A PROCESS FOR PREPARATION OF FABRIC HAVING A COATING OF A SYNTHETIC RESINOUS COMPOSITION WHICH RESISTS THE PASSAGE OF LIQUID WATER BUT PERMITS TRANSMISSION OF WATER VAPOR COMPRISING MIXING 15-60 PERCENT TO CROSS LINKED ORGANIC PIGMENTS CONTAINING AMIDE GROUPS BASED ON THE DRY COATING WITH AN ELASTOMER CARRIED IN A LIQUID MEDIUM CONTAINING 5-80 PERCENT SOLIDS CONTENT OF THE ELASTOMER, AND COATING ON A FABRIC. 